Personally I think 11 is a bit short. I have the Pedersoli howdah with 11 inch smooth 20ga barrels (and 58 cal rifles, but that's BTW) and it carries good dense pattern to 25~30 meters, but the velocity at that distance is insufficient to break clay.

I also have a Pedersoli SxS 12ga (muzzleloading). It has light "almost 29 inch" barrels and last time I took it to a clay shoot I could lhit from the cylinder barrel at 30m. I even got few lucky "long shots" from the improved barrel at 45-50m! If you asked me I would say it is impossible to hit at that distance, but it did happen few times :-)

My load for the 12ga is 80 grains of VesuvitLC (2f/3f mix equivalent), a Pedersoli thick(3/4 inch) cork wad. 1.25 ounce lead and a card wad on top. It shoots beautifully.